Detection of leaks in hydrocarbon storage systems



` Dec. 4, 1956 P. F. DOUGHERTY DETECTION OF LEAKS IN HYDROCARBON STORAGE SYSTEMS Filed March 19, 1954 ATTORNEY United States Patent() DETECTION OF LEAKS IN HYDROCARBON STORAGE SYSTEMS Patrick F. Dougherty, Chester Heights, Pa., assignor to Sun Oil Company, Philadelphia, Pa., a corporation of New `lersey Application March 19, 1954, Serial No. 417,433

3 Claims. (Cl. 73-40.5)

This invention relates to ra method for detecting leaks in tubing employed in the sub-surface storage of hydrocarbons, and more particularly relates to a method for detecting and locating leaks in tubing employed for introducing and removing brine from sub-surface salt cavities in processes for storing hydrocarbons in such cavities.

The storage of hydrocarbons, such as propane, in subsurface salt cavities is well-established. Such cavities are usually prepared by drilling to a point within a sub-surface salt formation, such as a salt stratum or salt dome, and dissolving salt therefrom by contacting with water circulated into the well, brine being removed from the system as water is introduced. Circulation of water and removal of brine is continued until a cavity of the desired capacity is obtained. A tubing,vwhichcan be the tubing employed for introducing water in the preparation of the cavity, is positioned within the borehole and provides uid communication between the lower portion of the cavity and the ground surface. The usual method of storing hydrocarbons in the cavity is to introduce the |hydrocarbon to be stored through the annulus between the borehole wall, or borehole casing, and the tubing positioned within the borehole. For brevity, this annulus, which provides fluid communication between thel ground surface and the upper portion of the salt cavity, is hereinafter designated simply as the annulus, or borehole annulus. The introduced hydrocarbon forms an upper layer over brine and as hydrocarbon is introduced, brine is displaced from the system'through the tub-ing. The stored hydrocarbon is recoveredV by introducing brine through the tubing, thereby forcing hydrocarbons to the surface through the annulus. In operating the process for hydrocarbon storage, brine is always maintained in the tubing regardless of whether hydrocarbon is being introduced, removed, or remaining in storage.

The presence of leak in the tubingcreates an extremely hazardous condition. For example, in the initial filling of a salt cavity with a hydrocarbon by introducing the hydrocarbon into the cavity through the annulus, the presence of la leak in the tubing results in introduced hydrocarbon owing through the leak and out of the system through the tubing in which brine alone normally flows. An extremely hazardous condition results vfrom the escape of quantities of propane, which in any event is lost to the operator. Leaks which cause such a ldangerouscondition can be initiated as cracks or pin holes present in thedrill pipe which are enlarged by'circulation of brine Vin 'the preparation of the cavity, or in subsequent movement of brine through the tubing inthe storage of hydrocarbons.

Leaks can also be through threaded connections between sections of tubing, or through ruptures in the tubing caused by a portion of the borehole wall collapsing.

In the rotary drilling of wells employngcontinuous circulation of fluid, such as drilling mud, methods for determining leaks in the drilling stem have been described. These methods are usually based upon continuous circulation of a fluid such asdrilling mud, and/or employ delicate pressure measurements, and/ or introduce foreign materials into the system, and hence are not suitable for testing tubing in the present instance.

An object of the invention is to provide a process for determining leaks in tubing employed as the brine conduit between the cavity and ground surface in the sub-surface storage of hydrocarbons. A fu-rther object is to determine the location of leaks in such tubing. Other objects will be apparent from the following specification.

It has now been found that by filling with brine the system used in the sub-surface storage of hydrocarbons, which system comprises the salt cavity, tubing and annulus, and thereafter introducing a quantity of propane into the tubing thereby displacing a quantity of brine through the annulus, discontinuing the introduction of propane for a time suicient to permit any propane passing by leakage from the tubing to the annulus to reach the surface of the brine in the annulus, and testing for the presence of propane in the upper portion of the annulus, the presence or absence of-a leak in the portion of the tubing filled with propane is established. A further quantity of propane is then introduced and the above steps repeated. The existence of propane in the annulus indicates a leak, and from a knowledge of the size of the tubing and the volume occupied by propane introduced, the location of the leak can be readily ascertained. By introducing relatively small increments of propane, thelocation of the leak can be ascertained with considerable accuracy. Where only a rough determination of the location of the leak is desired, a relatively large quantity of propane can be introduced to ll a relatively large section of the tubing which, of course, gives a more rapid but less accurate method for determining the location of a leak. The volume occupied by thel propane, which determines the level thereof in the tub-ing, is determined by measuring lthe volumeintroduced, or by measuring the volume of brine displ-aced thereby.

The accompanying gure is a more or less diagrammatic illustration of an assembly for performing the method of the invention and isA hereinafter discussed in detail. To illustrate the process of the invention, a sub-surface salt cavity of about 50,000 barrels capacity designed for storing hydrocarbonsis llled with brine. The annulus providing uid communication between the ground surface to the upper portion of the cavity and the tubing providing fluid communication between the ground surface and the bottom portion of the cavity are also lled with brine. A measured volume of liquid propane is then introduced into the tubing and an equivalent volume of brine is thereby displaced from the system through the annulus. The upper portion of the annulus is tested for prop-ane. If the test is negative an additional measured volume of propane is introduced into the tubing and the steps of testing for propane repeated. The detection of propane in the annulus after the introduction of Iany given increment of propane locates the leak withinthat portion of tubing from which brine was displaced by such increment.

Propane has been used in the foregoing description of the invention for convenience. Other ,hydrocarbons may be employed andit is advantageous to employ in the test the hydrocarbon to be stored. Good results are obtained, for example, with butane, pentane, hexane, hydrocarbon mixtures such as gasoline, and the like. lt is preferred to employ a hydrocarbon, or mixture of hydrocarbons, which can be introduced and maintained as a liquid within the tubing, but which on reaching the upper portion of the annulus, which is maintained substantially at atmospheric pressure, becomes a gas. Propane and butane are the 'preferred hydrocarbons to employ. I-t may be observed that in maintaining a hydrocarbon such as propane in the liquid phase'within the tubing, especially during the initial stages of operation, it is desirable to introduce the propane at a pressure regulated with respect to the rate brine is displaced from the system so that the pressure maintains the propane in liquid phase. The time required to permit any propane that may Vescape into `the annulus to rise to the upper portion thereof is insufficient for the propane to become gaseous. In the event normally liquid hydrocarbons are used, suchas gasoline, instead of testing the upper portion of the annulus for gas, it is tested by any convenient appropriate means for the liquid hydrocarbon or hydrocarbon mix-ture used. Hydrocarbons that remain gaseous when employed in the process, such as ethane, methane, and mixtures thereof and the like, can also be employed but are not preferred. When such gaseous hydrocarbons are employed, it is advantageous to measure the quantity of the eflluent brine displaced to determine the location of the gas-brine interface within the tubing rather than by measuring the volume of gas introduced. When the quantity of eflluent brine is measured, it is advantageous to operate the process with substantially saturated brine so that contraction of liquid volume, observed on increasing the salt concentration in brine, is not a factor. In any event, however, the contraction is small in relation to the volume of brine displaced and can be ignored in most operations.

Various methods can be employed for detecting the hydrocarbon which, in the event of a leak, appears in the upper portion of the annulus during the present process. Conventional type gas detectors give good results where the hydrocarbon is a gas under the conditions, which may include substantially atmospheric pressure, existing in the upper portion of the annulus. For example, a gas tran of a type known to the art can be employed and has the advantage, in some methods, of providing for the visual determination of the presence of gas.

Attention is now directed to the accompanying gure which is a more or less diagrammatic illustration of the process of the invention. Salt cavity 1, located in subsurface salt formation 2, is provided with fluid communication with the ground surface through tubing 4 and annulus 5, which `is the annulus between borehole casing 7 and tubing 4. ln the process of storing hydrocarbons, the hydrocarbon to be stored is introduced through conduit 6 and passes through annulus 5 into cavity 1. and in so doing displaces brine from the system through tubing 4. The hydrocarbon, being of lower specific gravity than, and substantially immisicible with, the brine, forms an upper layer over the brine layer. For recovery of the hydrocarbon, the flow is reversed, brine being introduced through tubing 4 to displace stored hydrocarbon annulus and conduit 6.

In testing for leaks in tubing 4 in accordance with the invention, the entire system is lled with brine. It is advantageous to make this test before using the cavity for hydrocarbon storage, since the system is at that time filled with brine from the preparation of the cavity. The process may, however, be employed at any time necessary or desirable. It is preferred to use brine substantially saturated with respect to salt. In the event the brine is not saturated, it can be circulated through the cavity until substantial saturation is reached. Propane, used to illustrate various materials which can be employed in liquid phase, is introduced into tubing 4 to a level represented by S. The rate of introduction is such that the propane is maintained in liquid phase. The location of level S is determined by measuring the volume of propane introduced, or by measuring the volume of brine effluent from conduit 6. For most applications, compressibility of the liquid propane, or other liquid hydrocarbon, can be ignored. A time suticient for any propane escaping from tubing 4 to annulus 5 to pass upwardly to the top of annulus 5 is allowed. The actual time required for propane escaping from the tubing to rise to the top of the annulus depends upon the depth of the leak, the difference in specific gravity of the brine and propane, and

the like, but will seldom be more than several minutes, and will under all practical conditions of operation be substantially within the range of from about l second to l0 minutes. The specific gravity of the brine can vary from about 1.003 to about 1.2, but will usually be from about 1.1 to 1.2, saturated brine having a specific gravity of about 1.2 under usual conditions of operation. The top portion of the annulus or equivalent portion of the system is then tested for propane. If none is found, an additional increment of propane is introduced through tubing 4, say to a level represented by 9, and the foregoing steps repeated. In the figure used to illustrate the invention, any propane escaping lthrough tubing 4 passes up through annulus 5 and is thereafter detected. lt is preferred to employ a gas trap such as represented by 10, it being understood that the gas trap can be connected with conduit 6 preferably adjacent borehole casing 7, or the like. Propane can be removed from trap 10 through conduit 12 by operating valve 11, and the presence thereof in conduit 12 can be established by any convenient means. It is advantageous in some instances to have at least a portion of gas trap 10 or its equivalent constructed of transparent material so that visual observation of gas therein can be made. This can be accomplished by having the entire trap 10, or a portion thereof, made of transparent material, such as glass or a transparent plastic, or by providing a sight glass (not shown) by conventional means. Where conduit 6 is at substantially the upper end of the annulus, the eluent brine can be tested for propane and good results obtained. The steps of in troducing propane and testing for leaks can be repeated until the entire length of tubing 4 is tested. Any suspected portion thereof can likewise be tested. An advantage of the present invention is that small increments of propane can be introduced in order to determine within relatively narrow limits the location of a leak. For example, if a large increment of propane locates a leak between levels 8 and 9, brine can be introduced through conduit 6 and annulus 5 to force propane out through tubing 4 until the level of the brine is above height 8. Propane is then introduced in relatively small increments until the leak, known to be between heights 8 and 9, is precisely determined.

In some instances, a tubing such as represented in the figure by numeral 4 is broken or damaged within the cavity such as by a falling Stringer. By stringer, as used herein, is meant a relatively narrow vein of substantially water insoluble material that remains suspended from the top or side of the cavity after salt is dissolved from around it. Such damage or breakage is determined in the process of the invention, and hence can be corrected prior to the introduction of large quantities of propane through the annulus.

Pumps, valves, gauges, and the like, the location and use of which are within the scope of those skilled in the art, have been largely omitted from the figure.

Example A salt cavity at a sub-surface depth of about 2300 feet having a capacity of about 50,000 barrels was prepared as above described. Before introducing propane for storage therein, it was desired to test for leaks the tubing to be used as the brine conduit in the storage process.

With the system, including the cavity, annulus and tubing lled with substantially saturated brine having a specific gravity of about 1.2, a measured volume of liquid propane, about 2 barrels, was introduced into the tubing. The tubing had an internal diameter of 4% inches, and hence the propane-brine interface in the tubing was lowered about 40.7 feet by each barrel of propane added. A time sufficient for propane to appear in the upper portion of the annulus, about 5 seconds, was permitted and the upper portion of the annulus tested for propane. The test was negative. Additional measured increments of liquid propane were introduced and the above procedure repeated, the measured increments of propane varying from about 1 to 3 barrels. A total of about 54 barrels of liquid propane, introduced in increments of from 1 to 3 barrels, completed the test. The time allowed for propane to rise to the upper portion of the annulus on introduction of the last increment of liquid propane was about minutes. In the actual test no leak was found, and hence the cavity was immediately placed in service for the storage of hydrocarbons by introducing brine into the annulus to force the propane from the tubing, and then introducing propane into the annulus for storage over brine as hereinbefore described. No diiculty from leaks was observed in the subsequent storage of propane.

The invention claimed is:

1. In the method of storing a hydrocarbon in a subsurface salt cavity having uid communication with the ground surface through a tubing positioned within the borehole and through the borehole annulus, said borehole annulus providing fluid communication between the ground surface and the upper portion of said cavity and said tubing providing fluid communication between the ground surface and the lower portion of said cavity, in which method the hydrocarbon to be stored is introduced into said cavity through said annulus to thereby displace brine from the system through said tubing, and in which method the stored hydrocarbon is recovered by introducing brine into said cavity through said tubing to thereby displace stored hydrocarbon from the system through said annulus, the method of detecting and locating leaks in said tubing which comprises the steps of: (1) lilling said cavity, said annulus and said tubing with brine, (2) introducing a quantity of hydrocarbon into said tubing sucient to displace a portion of the brine therefrom and from the system through said annulus, (3) discontinuing the introduction of hydrocarbon for a time suicient to allow hydrocarbon passing from a leak in said tubing to said annulus to rise to the upper end of said annulus, (4) testing for hydrocarbon at the upper end of said annulus, (5) repeating said steps (2), (3) and (4) until at least a portion of said tubing has been tested, whereby the existence and location of a leak in said tubing is determined.

2. Method according to claim 1 wherein the hydrocarbon introduced into tubing is a normally liquid hydrocarbon.

3. In the method of storing a hydrocarbon in a subsurface salt cavity having lluid communication with the ground surface through a tubing positioned within the borehole and through the borehole annulus, said borehole annulus providing uid communication between the ground surface and the upper portion of said cavity and said tubing providing uid communication between the ground surface and the lower portion of said cavity, in which method the hydrocarbon to be stored is introduced into said cavity through said annulus to thereby displace brine from the system through said tubing, and in which method the stored hydrocarbon is recovered by introducing brine into said cavity through said tubing to thereby displace stored hydrocarbon from the system through said annulus, the method 0f detecting and locating leaks in said tubing which comprises the steps of: (1) iilling said cavity, said annulus and said tubing with brine, (2) introducing a known volume of propane into said tubing sufiicient to thereby displace a portion of the brine therefrom and from the system through said annulus, (3) discontinuing the introduction of propane for a time suicient to allow propane passing from a leak in said tubing to said annulus to rise to the upper end of said annulus, (4) testing for propane at the upper end of said annulus, (5) repeating said steps (2), (3) and (4) until at least a portion of said tubing has been tested, whereby the existence and location of a leak in said tubing is determined.

References Cited in the le of this patent UNITED STATES PATENTS 1,693,737 Weldon Dec. 4, 1928 1,933,791 Crouch Nov. 7, 1933 2,164,195 Waltermire June 27, 1939 2,528,956 Hayward Nov. 7, 1950 FOREIGN PATENTS 421,324 Great Britain Dec. 18, 1934 OTHER REFERENCES Publication, Subsurface Storage Facilities for L. P. G. by Van Fossan, Oil & Gas Journal, Apr. 27, 1953, pgs. 192-198. 

1. IN THE METHOD OF STORING A HYDROCARBON IN A SUBSURFACE SALT CAVITY HAVING FLUID COMMUNICATION WITH THE GROUND SURFACE THROUGH A TUBING POSITIONED WITH THE BOREHOLE AND THROUGH THE BOREHOLE ANNULUS, SAID BOREHOLE ANNULUS PROVIDING FLUID COMMUNICATION BETWEEN THE GROUND SURFACE AND THE UPPER PORTION OF SAID CAVITY AND SAID TUBING PROVIDING FLUID COMMUNICATION BETWEEN THE GROUND SURFACE AND THE LOWER PORTION OF SAID CAVITY, IN WHICH METHOD THE HYDROCARBON TO BE STORED IS INTRODUCED INTO SAID CAVITY THROUGH SAID ANNULUS TO THEREBY DISPLACE BRINE FROM THE SYSTEM THROUGH SAID TUBING, AND IN WHICH METHOD THE STORED HYDROCARBON IS RECOVERED BY INTRODUCING BRINE INTO SAID CAVITY THROUGH SAID TUBING TO THEREBY DISPLACE STORED HYDROCARBON FROM THE SYSTEM THROUGH SAID ANNULUS, THE METHOD OF DETECTING AND LOCATING LEAKS IN SAID TUBING WHICH COMPRISES THE STEPS OF: (1) FILLING SAID CAVITY, SAID ANNULUS AND SAID TUBING WITH BRINE, (2) INTRODUCING A QUANTITY OF HYDROCARBON INTO SAID TUBING SUFFICIENT TO DISPLACE A PORTION OF THE BRINE THEREFROM AND FROM THE SYSTEM THROUGH SAID ANNULUS, (3) DISCONTINUING THE INTRODUCTION OF HYDROCARBON FOR A TIME SUFFICIENT TO 